Effects of Solar Radiation on Human Function and Cognition

Solar radiation is a physical phenomenon which interacts with any human being throughout its life on earth. This interaction is associated with numerous physiological functions, as well as a broad spectrum of deleterious effects on human health and wellbeing. The aim of this study is to elucidate the effects of solar radiation on human function and cognition in environmental conditions characterized by the same thermal stress.

Volunteers were exposed to four different environmental conditions [two hot environments: 30°C WBGT (1st: air temperature = 32.6°C; relative humidity = 30%; wind speed = 0.5 m/s; solar radiation = 800 W/m2 and 2nd. air temperature = 40.6°C; relative humidity = 30%; wind speed = 0.5 m/s; solar radiation = 0 W/m2) and two neutral environments: 20°C WBGT (1st: air temperature = 19.1°C; relative humidity = 30%; wind speed = 0.5 m/s; solar radiation = 800 W/m2 and 2nd. air temperature = 28.5°C; relative humidity = 30%; wind speed = 0.5 m/s; solar radiation = 0 W/m2], over an equal number of days. The aforementioned environmental scenarios were randomly allocated for each volunteer. To minimize participant bias, the true purpose of the study was hidden from the volunteers. Of course, once the data collection was completed, all volunteers were informed about the true purpose of the study and gave their permission to analyze and publish these data. During this blinded randomized controlled trial, all volunteers participated four times in an experimental protocol with a total duration of 03:20:00, following an hour of preparation and baseline tests. The protocol had three main time periods [rest (two hours), work (cycling) at 100 W (one hour), and recovery (twenty minutes)]. Anthropometrical data [age; body stature (Seca 213; seca GmbH & Co. KG; Hamburg, Germany); body mass (BC1000, Tanita corporation, Tokyo, Japan); body fat (DXA scan: Lunar DPX Madison, GE Healthcare, Wisconsin, USA); and lean mass (DXA scan: Lunar DPX Madison, GE Healthcare, Wisconsin, USA)] were collected two days prior to the experiments. Volunteers were euhydrated prior to the experiments. Hydration status was assessed using a handheld refractometer (ATAGO Ltd, Tokyo, Japan) and determined as either euhydrated (USG < 1.020) or dehydrated (USG ≥ 1.020) according to the current guidelines. Water consumption was restricted during the experimental protocol. The same clothing consisting of a light-blue t-shirt (100% cotton), a black exercise short pant (100% polyester), and a pair of medium-high socks (100% cotton) were used by volunteers throughout the experiments. A pair of sunglasses was mandatory to be worn throughout the experiments. Furthermore, experiments took place during the same hour of the day for each participant and an 8-hour fasting was ensured prior to the experiments. Furthermore, participants were requested to avoid caffeine and alcohol consumption for at least twelve hours before the experiments, as well as to avoid salt and sugar consumption eight hours before the experiments. During the study, continuous heart rate, core temperature, mean skin temperature, skin blood flow, and sweat rate were measured. Specifically, heart rate was collected using wireless heart rate monitors (Polar Team2. Polar Electro Oy, Kempele, Finland). Core temperature was collected using telemetric capsules (BodyCap, Caen, France). Skin temperature from four sites was measured using wireless thermistors (iButtons type DS1921H, Maxim/Dallas Semiconductor Corp., USA) and was expressed as mean skin temperature according to Ramanathan [mean skin temperature = 0.3(chest + arm) + 0.2(thigh + leg). Skin blood flow was measured with a laser Doppler flowmeter (PeriFlux 4000, Perimed, Stockholm, Sweden) at right forearm (brachioradialis) and leg (gastrocnemius). The probe (PROBE 413 Integrating Probe, Perimed, Stockhold, Sweden) was held in place with a plastic holder (PH 13, Perimed, Stockhold, Sweden). Sweat rate was measured at three regions [forehead, thigh (quadricep), and arm (bicep)] using ventilated capsule method. Thermal comfort (1 = comfortable; 5 = extremely uncomfortable), thermal sensation (-3 = cold; +3 = hot), and perceived exertion (6 = no exertion at all; 20 = maximal exertion), alongside cognitive performance (vigilance, divided attention, memory test, and reaction time) were assessed at baseline, pre work condition (01:40:00), and post work (03:00:00) conditions. Volunteers entered the chamber five minutes prior to the experiments to install data loggers and accompanied sensors on their body.

Exposure to hot environment (30°C WBGT) with radiation (800 W/m2) Exposure for 03:20:00: rest (two hours) work (cycling) at 100 W (one hour) recovery (twenty minutes)

Exposure to hot environment (30°C WBGT) with radiation (0 W/m2) Exposure for 03:20:00: rest (two hours) work (cycling) at 100 W (one hour) recovery (twenty minutes)

Exposure to neutral environment (20°C WBGT) with radiation (800 W/m2) Exposure for 03:20:00: rest (two hours) work (cycling) at 100 W (one hour) recovery (twenty minutes)

Exposure to neutral environment (20°C WBGT) with radiation (0 W/m2) Exposure for 03:20:00: rest (two hours) work (cycling) at 100 W (one hour) recovery (twenty minutes)

Exposure to hot environment (30°C WBGT) with radiation (800 W/m2) Exposure for 03:20:00: rest (two hours) work (cycling) at 100 W (one hour) recovery (twenty minutes)

Exposure to hot environment (30°C WBGT) with radiation (0 W/m2) Exposure for 03:20:00: rest (two hours) work (cycling) at 100 W (one hour) recovery (twenty minutes)

Exposure to neutral environment (20°C WBGT) with radiation (800 W/m2) Exposure for 03:20:00: rest (two hours) work (cycling) at 100 W (one hour) recovery (twenty minutes)

Exposure to neutral environment (20°C WBGT) with radiation (0 W/m2) Exposure for 03:20:00: rest (two hours) work (cycling) at 100 W (one hour) recovery (twenty minutes)

Heart rate data were continuously monitored using a Polar Team system (Polar® Team 2, Polar Electro Oy, Kempele, Finland

Temperature at the skin surface area was continuously monitored using iButton sensors (type DS1921 H, Maxim/Dallas Semiconductor Corp., USA)

Core body temperature was assessed throughout the entire protocol using telemetric capsules (e-Celsius, BodyCap, Caen, France) that was given to the workers to ingest prior to their work shift

Skin blood flow was measured throughout the entire protocol with a laser Doppler flowmeter (PeriFlux 4000, Perimed, Stockholm, Sweden) at right forearm (brachioradialis) and leg (gastrocnemius)

Sweat rate was measured throughout the entire protocol at three regions [forehead, thigh (quadricep), and arm (bicep)] using ventilated capsule method

Participants were assessed at baseline and every half an hour up until the end of the protocol at "hour 3".

Vigilance was assessed via vigilance test. In this task two squares arranged vertically. A pattern jumps from one square to the other. Sometimes the pattern remains in one square. In this case volunteers had to touch the screen of a tablet computer as fast as possible. The total duration for the current task was set to 6 minutes. This is a well-known vigilance test described in the Test for Attentional Performance and previously used in the literature.

Thermal comfort was assessed via the thermal comfort scale (1 = comfortable; 5 = extremely uncomfortable).

Participants were assessed at baseline and every half an hour up until the end of the protocol at "hour 3".

Participants were assessed at baseline and every half an hour up until the end of the protocol at "hour 3".

Divided attention task incorporates both auditory and visual stimuli in parallel. During the test a number of visual stimuli (crosses) appear in a random configuration in a 4X4 matrix. At the same time volunteers hear high and low beeps in a random order. The task is to detect whether crosses form a square and at the same time two high or low beeps are emitted twice in a row. This is a well-known vigilance test described in the Test for Attentional Performance and previously used in the literature.

This is a computerized version of Sternberg's memory task. In this task volunteers had to observe carefully a random sequence of one to six digits (0 to 9) painted in white and displayed for 1.2 seconds each. Thereafter, a two second delay was followed by a random test digit filled in yellow color. Participants had to response as fast as possible by pressing "yes" or "no" button to indicate where the yellow digit appeared in the previous sequence or not, respectively. In this test participants were tested in 24 trials.

In this task participants had to respond as quickly as possible, by untouching the screen, after receiving a visual or acoustic stimulus.

Inclusion Criteria: Healthy non-smoker individuals who exercise at least three times per week Exclusion Criteria: Non healthy individuals and smokers